Arianism is a heresy of early Christianity involving the nature of Jesus Christ. Arianists denied that Jesus Christ and God the Father were one, seeing them as different Divine entities. The conflict between Arianism and traditional trinitarianism was the first important doctrinal difficulty in the Church after the legalization of Christianity took place under Emperor Constantine I, and ended with Arianism being declared a heresy.

Arius[?] was a Christian priest in Alexandria, Egypt. In A.D. 321 he was condemned by a synod at Alexandria for teaching a heterodox view of the relationship of Jesus Christ to God the Father. Arius himself died without repudiating his doctrine. Arius and his followers agreed that Jesus was the son of God, but denied that they were one substance (Greek: homo-ousios). Instead, they viewed God and the Son as having distinct but similar substances (Greek: homoi-ousios). The difference in Greek was literally one iota or "letter i" of difference. Jesus is, for Arianism, inferior or subordinate to God the Father. The specific summary statement that was rejected by the councils, is that "there was a time when Jesus Christ was not"; the rejected statement meant that Jesus was a created being, rather than being coeternal with the Father and the Holy Spirit. At issue was the doctrine of the Trinity.

Because Arius and his followers had great influence in the schools of Alexandria - predecessors of modern universities or seminaries - their theological views spread, especially in the eastern Mediterranean. By 325 the controversy had become significant enough that Emperor Constantine I called an assembly of bishops, the first Ecumenical council at Nicaea, (modern Iznik, Turkey) (the First Council of Nicaea). The arguments that prevailed at Nicaea were formulated in the Nicene Creed, which is still recited in Catholic, Orthodox, and some Protestant services. Emperor Constantine ordered Arius exiled and the Arian books to be burned.

Despite the decision of the Council of Nicaea, Arianism not only survived but flourished for some time. The patronage of members of the imperial family allowed Arian bishops to rule in many centers. Having never converted any sizeable group of the laity, Arianism had died out inside the Empire by the 380s; it was debated and rejected again by the Second Ecumenical Council in Constantinople in 381.

However, during the time of Arianism's flowering in Constantinople a missionary named Ulfilas was sent out to the Gothic barbarians across the Danube River. His initial success in converting this Germanic people to an Arian form of Christianity was strengthened by later events. When the Germanic peoples entered the Roman Empire and founded successor-kingdoms, many of them used their Arian religion to differentiate their people from the local inhabitants and maintain their group identity against the Catholic population. See: Ostrogoths, Visigoths, Vandals, Burgundians, Lombards. By the 8th century assimilation had ended any surviving Arian churches. Only the Franks among the Germanic peoples entered the empire as pagans and converted to Catholic Christianity directly.

The modern Jehovah's Witnesses and the Church of Jesus Christ of Latter-day Saints have beliefs similar, but different from those of Arius. Jesus is seen as subordinate to God the Father (e.g., he acts on his Father's wishes), but the primary teaching is that as they are both perfect and free from sin, there is no possibility of a disagreement between them.

Several historical novels involve the turning-point of Western history in which the tolerant, practical flexibility of Arianism in the Gothic and Celtic Churches was overpowered by the intolerant, centralized dogma of the Roman Church. Some of these novels are excellent, such as "Raptor", by Gary Jennings; some are simply awful, such as "The Da Vinci Code", by Dan Brown. A science-fiction novel in which a return to Arianism is the last best hope of an exhausted post-Christian West remains to be written -- though Mormon s-f writers like Gene Roddenberry and Orson Scott Card have made a start.